Flash floods have killed more than 900 people in the two countries and many more are missing as rain still falls.
The race is also on to recover bodies floating in the water to stop diseases spreading throughout the region.
About 300 bodies have been found in the Haitian town of Mapou, under so much water it has been described as a lake.
 |  It is impossible to reach that whole area unless you have a helicopter  Marko Kokic Red Cross |
He told the World Today programme that aid workers were trying to get hold of small boats so that volunteers could help recover the bodies.
Another key priority, Mr Kokic said, was to move the survivors out of the danger zones in case there was more flooding, and to send in water purification tablets, shovels, and body bags.
Disease fears
The flash floods have destroyed homes and washed away roads in towns and villages across Haiti and the Dominican Republic, which make up the Caribbean island of Hispaniola.
Two teams of United Nations disaster relief experts arrived on the island on Friday to help local UN relief teams and other aid workers.
[an error occurred while processing this directive] “They are there to do a rapid evaluation of the situation and the needs on the ground,” said Elisabeth Byrs, a Geneva-based spokeswoman for the UN Office for the Co-ordination of Humanitarian Affairs.
“We believe there are more than a total of 48,000 people affected in the two countries.”
On Thursday, three helicopters carrying water, chlorine purification tablets and other supplies were sent to Mapou and Fond Verettes, where 165 people are said to have died.
Another 100 bodies have been found in the Haitian coastal town of Grand Gosier.
Over the border, disinfectant is to be sprayed in the Dominican Republic town of Jimani to stop the spread of disease.
US-led peacekeeping forces – who were sent to Haiti to help keep order after the February overthrow of President Jean-Bertrand Aristide – have been helping the relief effort.
Many flood-hit towns are impossible to reach except from the air. And continuing rain has hampered the access even of helicopters.
Even before the flooding, Mapou was said to take three or four hours to reach from Jacmel, the nearest city.
Rivers of mud
Across the border in the Dominican town of Jimani, rescuers continued to dig through rivers of mud, reported AFP.
The town – now a muddy scar in the landscape, after the flood waters drove through – was the worst-hit in the Dominican Republic.
At least 329 bodies have been found there, and at least 300 people are missing.
Dominican President Hipolito Mejia and the US ambassador, Hans Hatler, have toured the stricken town.
Mr Hatler announced $50,000 in aid, saying more would be delivered as soon as possible.
Nearly 700 Red Cross volunteers were reportedly in the town, helping those injured and putting up mosquito nets to protect against outbreaks of malaria and dengue fever.